SMART Currently unreadable sectors

I have the smartd daemon running on a 12.10 server with raid 6 array using mdadm and a few days ago I got an email about bad sectors on one of the disks:

Code:

Device: /dev/sdd [SAT], 8 Currently unreadable (pending) sectors

immediately followed by another email:

Code:

Device: /dev/sdd [SAT], 8 Offline uncorrectable sectors

I ordered a replacement disk which is yet to arrive. /dev/sdd has not yet been kicked out of the array. Anyway these emails have since continued and reported the same number of unreadable sectors. Last night I got another couple of emails:

Code:

Device: /dev/sdd [SAT], 65528 Currently unreadable (pending) sectors

and again immediately followed by:

Code:

Device: /dev/sdd [SAT], 65528 Offline uncorrectable sectors

However when I run 'smartctl -a /dev/sdd' it doesn't appear to show any issues other than the original extended tests failing. I don't really know how to read smartd output. But it appears the sectors are no longer reported as an issue but also haven't been reallocated as Reallocated_Sector_Ct is 0. But also the last few tests have completed succesfully, so I'm not sure why smartd generated last nights email. I'm going to replace the disk as soon as the replacement arrives but I am confused about the SMART output. Can anyone help me understand what's going on with this please?

Re: SMART Currently unreadable sectors

Re: SMART Currently unreadable sectors

Originally Posted by ahallubuntu

Looks like a glitch or a false alarm. Originally you had 8 pending sectors - then 65528 (which in hex is FFF8 unsigned or -8 signed decimal). Looks too much like a glitch to have first 8 then -8 sectors showing up vs. real numbers.

Wow, good catch. I didn't notice that. 8 to -8 seems unlikely to be a coincidence to me and a glitch seems likely.

Originally Posted by ahallubuntu

I'd make sure the extended test passes on sdd anyway. It goes without saying you should be making regular backups anyway, even with a RAID. A RAID protects you against only hard drive failure, not file system corruption or accidental erasure.

The previous extended tests failed at 50% and the current test has only 10% remaining, so it's looking good so far. The system backs up important files using Crashplan but I don't have the bandwidtch to back up the 4TB of media to the cloud. Never sure how best to back up such a huge amount of data.

Originally Posted by ahallubuntu

FYI, pending sectors (when not false alarms) don't always become reallocated - that's how it's SUPPOSED to work but sometimes they get "stuck" and can never be re-allocated - and you are kind of out of luck. Sometimes writing zeros to the disk will clear them, usually not.

Understood, and I did read that md handles bad sectors by building the data from the other disks and attempting to write it back to the bad sectors; removing the disk from the arrays if this fails. So, I ordered a disk to replace it in anticipation of this happening but I guess it hasn't needed to read or write to those sectors. Or maybe it has and that's what reset them. Either way, I guess I shouldn't rush to replace the disk and just keep an eye on it?